If you are new to hostmonster hosting, definately you need to check on their hosting features before signing up with them. Chosen the wrong hosting plan is the most painful experience, and require some migration work that might take your whole day of time. Selecting the right plan, does safe your life 
Luckily hostmonster is offering both php version 4 and php version 5. And my hosting with hostmonster is still using the php4, and there is no current plan to upgrade to php5. As the wordpress script that i using now is require only php 4 and above.
If you need to run some custom script etc, do check on their system requirement. And selecting the right hosting features. If you come across that require php 5, you can contact support to move your site over to php 5 server. Just email to support and request for php 5 upgrade. They will do the remaining work. Simple as that.
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